The present invention relates to a method of assigning mobile stations to radio channels in a mobile radio communications system. The radio channels are assumed to be so-called multi-user channels, in other words a plurality of mobile stations may share a channel over a given period of time.
The invention also relates to an arrangement and to a method for assigning multi-user channels to mobile stations in accordance with said method.
In conventional radio communications systems for telephony, each unit is assigned its own channel with separate uplink and downlink. A given channel, represented for instance by a given time slot on a special carrier frequency, of a given base station cannot be assigned to a new mobile station before an earlier call connection on the channel has been released.
In the case of single-path non-selected transmission within the radio communications system, so-called broadcasting, a pool of available channels is utilized for transmitting messages with several simultaneous receivers. Many different algorithms for determining which of the available channels is most suitable for distributing this type of message have earlier been described. U.S. Patent Specification U.S. Pat. No. 5,530,917 teaches a method of optimizing channel utilization within a mobile telephony system. According to this method, channel assignment is effected on the basis of calculated load factors that have been obtained from at least two alternative sets of parameter values. The parameters include both radio-specific characteristics and other characteristics relevant to the call setup concerned. The method is applied both when changing channels for a call connection that has already been established and with handover.
International Patent Application WO-A1 96/07287 teaches a method of assigning carrier frequencies and time slots to mobile stations in a radio communications system of the TDMA/FDMA hybrid type (TDMA=Time Division Multiple Access. FDMA=Frequency Division Multiple Access). Mobile stations are assigned time slots on carrier frequencies such that mobile stations having roughly the same power output demand from the base station will be given the same carrier frequency. In consequence, the output power for each carrier frequency utilized is minimized and the available frequency band is used as effectively as possible. Interference in the radio communications system is also reduced.
International Patent Application WO-A1 95/07012 teaches a method in which base stations that encounter interference problems on certain of their channels are collected in a database. The base stations in the database are controlled to lower the output power on the channels concerned to a lowest value acceptable for the mobile stations that use these channels. An effort is thus made to constantly keep the number of base stations in the database as small as possible, therewith leading to a reduction in the interference in the radio communications system as a whole.
The service GPRS within GSM enables several mobile stations to use the same channel simultaneously for the transmission of information between a given base station and the mobile stations that are connected thereto (GPRS=General Packet Radio Service; GSM=Global System for Mobile communication). Each channel includes an uplink for the transmission of information from the mobile station to the base station, and a downlink for the transmission of information from the base station to the mobile station. The simultaneous use of a channel will mean that two or more mobile stations transmit and receive information on the channel in parallel, or that two or more mobile stations use the channel alternately over a given time period either to send information to a base station or to receive information from a base station. The downlink of a channel can be used typically to transmit information to a first mobile station at the same time as a second mobile station transmits on the uplink of the channel.
The information transmitted consists typically of data packets that are divided into information blocks of specific size. By reading a USF, different mobile stations of a limited number of stations can receive different information from one and the same information block, which is transmitted via a common physical channel (USF=Uplink State Flag). A USF, which is transmitted in the payload on the downlink of a channel and read by all mobile stations that use a specific channel, indicates to which mobile station the next-arriving information block is addressed and which mobile station may transmit information on the channel uplink in the next-arriving time slot.
GPRS pays no attention to individual communication parameters of the mobile stations when assigning a channel that is common to two or more mobile stations. The mobile station that has the highest output power requirement from the base station determines the output power level from the base station on the channel concerned. If the mobile station or the other mobile stations that share the channel places/place considerably lower output power demands on the base station, the base station will transmit with an output power that falls markedly below the minimum output power required, at least for some of the time. When mobile stations that are located on very different directional bearings, as seen from the base station, are assigned the same channel, there are essentially three different ways of making the channel available to all of these mobile stations at one and the same time. (1) A separate antenna lobe may be aimed at each mobile station; (2) an angular power spectrum that is based on information concerning both the directional bearing to the mobile station and the propagation conditions for the radio waves between the base station and the mobile station may be generated for each mobile station, and; (3) there may be used an antenna lobe that is sufficiently wide to cover all mobile stations. In all of these cases, and particularly in the latter case, electromagnetic energy will be transmitted over a wider geographical area than is necessary in order to send information from the base station to the mobile stations.
In the method described in WO-A1 95/07012, the power on channels that have already been assigned is minimized. However, the method does not address how the assignment of mobile stations to the base stations should be carried out with the view of avoiding interference in a radio communications system.
The method taught by WO-A1 96/07287 results in mobile stations that have similar output power demands being assigned different channels on the same carrier frequency. However, the method provides no solution to the problem of how a channel should be assigned to several simultaneous users of the channel. Furthermore, no account is taken of how the mobile stations are orientated in relation to the base station or to the length of time over which the mobile stations ask for channel capacity. Neither is this taken into account when optimizing channel usage as disclosed in U.S. Pat. No. 5.530,917.
The present invention provides a solution to the problem of optimally assigning several simultaneous mobile stations to the channels of a base station. By optimal assignment is meant here an assignment in which the total energy transmitted in sending information from the base station to all mobile stations within its radio coverage area is minimized. One link in minimizing the energy transmitted may, for instance, be achieved by minimizing the output power on each channel from the base station.
Thus, one object of the present invention is to reduce the total interference in a radio communications system.
Another object of the invention is to optimize the extent to which the channels of a base station are utilized with respect to current communication requirements and to predicted future communications requirements.
According to one embodiment of the inventive method, there are registered base-station specific parameters which describe properties of mobile stations in the proximity of the base station. The parameters may denote the output power demanded by respective mobile stations of the base station, the effective transmission time span for transmission of the information to be transmitted, a positional bearing relative to the base station (that is to say, the sector of the angular power spectrum of the base station in which the mobile station is located), the direction in which the mobile moves and/or its speed. An algorithm in the form of, e.g., cost functions is then applied to the parameter values. A decision is then made on the basis of the result of the algorithm as to how the mobile stations in the proximity of the base station shall be assigned to the channels of the base station, such that the total energy transmitted in sending information from the base station to the mobile stations is minimized. Finally, the mobile stations are assigned channels in accordance with this decision. If the available parameter values do not provide sufficient basis for a precise determination of the energy required to transmit said information, this energy is, instead, predicted with the aid of available parameter values.
A method according to this embodiment of the invention has the characteristic features set forth in Claim 1.
In one advantageous variant of the embodiment of the invention described above, all channels of the base station are assigned to all mobile stations within the radio coverage area at specific time intervals, in other words also to connections that have already been established between mobile stations and the base station. Assignment of mobile stations to the channels of the base station can thus be changed during the ongoing transmission of information.
In another advantageous variant of the aforesaid embodiment of the invention, a prediction is made as to which mobile stations will be located within the area covered by the base station within a specific future. This prediction is based on information relating to the speed vectors of the mobile stations, i.e. their direction and speed. Setting-up of future predicted mobile stations may include mobile stations that at that particular moment are located in adjacent cells (on their way into the cell concerned) and need not necessarily include all of the mobile stations that are located within the coverage area of the base station at that particular time (it is judged that these mobiles will leave the cell concerned within a short period of time). The prediction can also take into account access requests from respective paging messages to totally new (i.e. not earlier observed) mobile stations located within the area covered by the base station. Such predictions are based on call history, where it is assumed that new call connections can be described as stochastic processes that are Poisson-distributed, for instance.
The invention also relates to a method of assigning multi-user channels to mobile stations. The method involves registering specific parameters that describe properties of mobile stations in the proximity of a given base station. The parameters may denote the output power demanded of respective mobile stations from the base station, the effective transmission time for the transmission of information to be transmitted, positional bearings in relation to the base station, the direction in which respective mobile stations are moving and/or their speed. In accordance with the inventive method, the mobile stations are allocated channels from the list of channels held by the base station on the basis of the values of these parameters, so as to minimize the total energy transmitted in sending information from the base station to the mobile stations.
A method according to this embodiment of the invention has the characteristic features set forth in Claim 20.
The invention also relates to an arrangement for assigning multi-user channels to mobile stations. The arrangement includes a control unit which registers specific parameters that describe properties of mobile stations in the proximity of a given base station. The parameters may denote the output power demands of respective mobile stations on respective channels from the base station, the effective transmission time span for the transmission of the information to be transmitted, positional bearings in relation to the base station, position, changes in registered power, changes in output power demands from the base station, direction of movement and/or speed. The arrangement allocates channels from the list of channels held by the base station to the mobile stations in accordance with the inventive method and on the basis of the values of these parameters.
An arrangement according to this embodiment of the invention has the characteristic features set forth in Claim 24.
According to one advantageous embodiment of the invention, the arrangement includes a control unit which generates state flags which for each channel and time frame indicate which mobile station is authorized to send information to the base station.
The invention is effective in minimizing the total energy transmitted from base stations when sending information to associated mobile stations. Since the energy transmitted is equivalent to the sum of the output power during the effective transmission time span, the average electromagnetic power transmitted will be minimized and the total interference in the radio communications system therewith also reduced. This enables each available channel in the system to be utilized in the most effective manner possible, which, in turn, imparts a high degree of efficiency to the radio communications system as a whole.